This application claims the priority of German application No. 199 60 430.4, filed Dec. 15, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for operating an exhaust-gas cleaning system which comprises a nitrogen oxide adsorption accumulator and a sulphur oxide trap and to an exhaust-gas cleaning system which can be operated using this method.
Exhaust-gas cleaning systems with a nitrogen oxide (NOx) adsorption accumulator, such as an NOx adsorber catalytic converter, are used in particular in motor vehicles with an internal-combustion engine in order to minimize the nitrogen oxide emissions. In operating phases involving conditions which are unfavourable to nitrogen oxide reduction, such as in lean-burn mode, it is known for nitrogen oxide to be temporarily stored in the NOx accumulator by an adsorption process. This stored nitrogen oxide can then be converted back to nitrogen in a suitable subsequent reduction operating phase, such as in rich-burn mode. The terms lean-burn and rich-burn mode are understood to mean operation with oxygen-rich and low-oxygen exhaust-gas composition, respectively, for example as a result of an engine being operated with a lean or rich engine air ratio (i.e., with an air/fuel ratio for the air/fuel mixture burnt in the engine which is above or below the stoichiometric value).
The SOx trap prevents sulphur poisoning of the NOx adsorber. This is because, particularly when the engine is in lean-burn mode, sulphur which is contained in standard fuels and engine oils leads to sulphur dioxide being present in the exhaust gas. The sulphur dioxide is taken up by the NOx adsorber as a result of sulphates being formed, thereby reducing the capacity of this adsorber to store NOx. The SOx trap prevents this by trapping the sulphur oxides before they reach the NOx adsorption accumulator. It is subjected to a desulphurization treatment at the latest when its SOx storage capacity is exhausted. To achieve effective desulphurization, it is known to set elevated exhaust-gas temperatures of, for example, over 600xc2x0 C. and a rich exhaust air ratio, i.e. a ratio which is below the stoichiometric level.
A further special method for periodically desulphurizing a nitrogen oxide or sulphur oxide accumulator of an exhaust-gas cleaning system using secondary-air supply means is described in German patent application 199 22 962. In this method, during part of the desulphurization period, the exhaust-gas cleaning system is operated in such a way that, after a predeterminable desulphurization temperature has been reached, the accumulator air ratio (i.e., the air ratio in the exhaust gas supplied to the accumulator which is to be desulphurized), is made to oscillate between an oxidizing atmosphere, on the one hand, and a reducing atmosphere, on the other hand, by correspondingly changing the amount of secondary air supplied.
Patent DE 197 47 222 C1 describes a method for operating an exhaust-gas cleaning system in which on demand, from time to time, an NOx accumulator catalytic converter is desulphurized, i.e., accumulated sulphate is removed. The exhaust-gas cleaning system comprises secondary-air supply means having a secondary-air line and a secondary-air pump which can be controlled, for example, by an engine management system.
A problem with the conventional periodic regeneration of the SOx trap is that the NOx accumulator catalytic converter which follows it is exposed to the sulphur components released from the SOx trap, and in unfavourable circumstances this may lead to its ability to store NOx being partially blocked by the accumulation of sulphates. DE 198 02 631 A1 discloses an exhaust-gas cleaning system in which this problem is combatted by providing a bypass around the NOx accumulator catalytic converter in the exhaust system, via which bypass the exhaust gas is guided past the NOx accumulator catalytic converter while the SOx accumulator catalytic converter is desulphurized.
The present invention is based on the technical problem of providing an exhaust-gas cleaning system and an operating method which enable the SOx trap to be desulphurized with the minimum possible outlay and without there being any risk of sulphur poisoning of the nitrogen oxide accumulator catalytic converter.
In the method according to the present invention, the direction of flow of the exhaust gas during the desulphurizing phases is selected to be different from during the normal operating phases. During the normal operating phases, the exhaust gas which is to be cleaned is passed firstly via the SOx trap and then via the NOx adsorption accumulator. The sulphur oxide constituents of the exhaust gas, which are undesirable in the nitrogen oxide adsorption accumulator, remain trapped in the SOx trap, provided that the storage capacity of the SOx trap has not yet been exhausted and there is as yet no saturation behaviour. If the desulphurization phase is selected, the exhaust-gas flow is switched over in such a way that it passes through the two exhaust-gas cleaning components, namely the SOx trap and NOx adsorption accumulator, in the reverse direction (i.e., is passed firstly via the NOx adsorption accumulator and then via the SOx trap). This prevents sulphur compounds released during the desulphurization of the SOx trap from polluting the NOx adsorption accumulator.
In an embodiment of the present invention, secondary air is fed into the exhaust gas downstream of the SOx trap during the desulphurization phase. The secondary air is used to oxidize undesirable sulphur compounds, such as H2S and COS, which are released in the SOx trap, for which purpose an oxidation catalytic converter is connected downstream of the SOx trap.
An exhaust-gas cleaning system is equipped with an SOx trap and an NOx adsorption accumulator and can be operated according to the present invention. For this purpose, it has means for reversing the direction of flow of the exhaust gas in such a manner that the exhaust-gas stream can optionally be passed firstly through the SOx trap and then through the NOx adsorption accumulator or, conversely, firstly through the NOx adsorption accumulator and then through the SOx trap.
A configuration of the exhaust-gas cleaning system according to the present invention provides, as further components, an oxidation catalytic converter and secondary-air supply means.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.